METHODS OF COMPUTING FOR FURNACES 75
cold body. Combustion occurs with cold air and solid fuel
burned in a thin layer upon a simple bar grate. As an example of
such a construction, the firebox of a tubular boiler may be taken.
Combustion is effected rapidly under very unfavorable conditions
—the air supply is usually double that theoretically required.
2d Case.—A simple firebox, with a grate, but with a simple
brick arch over the fuel bed. Combustion occurs in a thick bed
of coal. In the case of the furnace working with chimney or
natural draft, it is necessary to have sufficient air pressure to
overcome the resistance of the fuel bed to the passage of the air
and gases.W Combustion is rapid, as in the preceding case, but is
effected under more favorable conditions. A thick bed of coal is
an excellent medium of combustion; the firebox covered with an
arch makes a satisfactory combustion chamber. Practice has
shown that, under these conditions, when the fire is well operated,
the combustion of coal may be effected with an air supply of about
1.50 times that theoretically required.
A furnace fired in this manner in the Lougansk works, using
a blower, burns 200 kg of coal per hour per square meter of grate
surface. When fireboxes of this type are used with natural or
chimney draft only, such a high rate of firing cannot be used; the
fuel consumption for such cases should not exceed 70 kg per hour.
For the ordinary types of furnaces with a firebox of this kind,
such as puddling furnaces, roverberatory furnaces for the melting
of iron, copper, etc., and for reheating furnaces, the computation
should be based on an air supply 1.50 times that theoretically
required, and with very good coal 1.70 times.
3d Case.—Producer-gas-fircd furnaces. These should be
figured as having a secondary air supply 1.50 times that theoretic-
4th Case.—Regenerative or Siemens furnaces. These should
be computed for a secondary air supply of 1.25 to 1.50 times the
5th Case.—Annealing furnaces, tempering furnaces, chamber
brick kilns, etc., should be computed for the theoretical air supply.
(1) This was mentioned in an earlier chapter. This pressure varies with the
height between the grate and the hearth of the furnace. The simplest method
of providing this pressure is by lowering the grate until it is sufficiently far
below the hearth to provide the pressure required to overcome the resistance
of the fuel bed.